Cashu

Cashu is an open-source ecash protocol built on Chaumian blind signatures that lets custodial mints issue bearer tokens redeemable for bitcoin, typically over the Lightning Network. Created by the developer known as calle in 2022, it revives the blind signature scheme David Chaum published in 1982, so a mint cannot link the tokens it signs to the users who later spend them.

Why it matters

Cashu trades custody risk for privacy and usability. Tokens are just data, so payments are instant, free, and possible even offline, and the mint learns nothing about who pays whom. The cost is that the mint holds the actual bitcoin, so users are exposed to loss or fraud exactly as with any custodian. The design is deliberately positioned for small balances and everyday spending, complementing rather than replacing self-custodied bitcoin.

How it works

A user deposits satoshis with a mint, usually via a Lightning payment, and the wallet presents blinded messages for the mint to sign. The mint signs without seeing the token contents and the wallet unblinds the signatures, producing bearer tokens in fixed denominations. Tokens pass between users like digital banknotes, and anyone holding valid tokens can redeem them at the mint for bitcoin. Because spent tokens are recorded only as anonymous secrets, double spending is prevented without identifying anyone.